1. Field of the Invention
The present invention relates generally to signal analysis and seismic data processing for geophysical surveys and particularly a method of separating and synthesizing seismic signals into harmonic and other components.
2. Description of the Related Art
The seismic exploration industry uses acoustic impulses to impart sonic vibrations into the earth to delineate subsurface structure for mineral exploration and development. These acoustic impulses may be from an explosive, implosive or swept-frequency (or chirp) source. A recording of the acoustic reflection and refraction wavefronts that travel from the source to a receiver is used to produce a seismic field record. Variations in the travel times of reflection and refraction events in these field records indicate the position of reflection surfaces within the earth. The analysis and correlation of events in one or more field records in seismic data processing produces an acoustic image that demonstrates subsurface structure. The acoustic images are used to find valuable mineral deposits.
The swept-frequency or chirp type seismic source may utilize a relatively long pilot signal such as 2 to 15 seconds to assure sufficient energy is imparted to the earth. The swept-frequency or chirp type source method relies on signal compression to compress the signal and ensure sufficient vertical resolution to resolve the position of subsurface reflectors. Signal compression generally is called deconvolution, with many techniques well known in the art of seismic data processing. Deconvolution of sweep or chirp signals compresses the source signal into a much shorter signal representative of a subsurface reflective boundary. The accuracy and effectiveness of any deconvolution technique is directly related to how well the source signal is known or understood. Most deconvolution operators are derived from statistical estimates of the actual source waveform.
In practice, the swept-frequency or chirp source waveform that is actually generated and imparted to the earth often does not resemble the idealized source signal the practitioner attempted to induce into the earth. Nonlinear effects in the earth and the source energy generating system, for instance a seismic vibrator, may create strong harmonics and other dynamic phenomena that introduce unpredictable distortions into the actual signal generated and imparted into the earth. These nonlinear effects are not predictable and are source site dependent. The strength and characteristics of the nonlinear distortions change from one source sweep position to the next. These nonlinear effects often interfere with the deconvolution enough to substantially degrade the acoustic images. A method that accurately decomposes the swept-frequency signal actually generated into its various components, allowing more effective deconvolution and other filtering, is highly desirable.
There will be many advantages to accurately decomposing seismic signals into their component harmonic parts. Deconvolution, a technique dependent on accurate signal analysis, will be much more accurate. Various separable harmonics or noise components of seismic signals can be enhanced, isolated or suppressed using appropriate filters. For instance, instrument noise, extrinsic noise and other noise not amenable to conventional filtering may be targeted precisely in many circumstances as a result of this decomposition method.
The present invention is a method for decomposing and synthesizing a seismic signal into harmonic and other extrinsic components that is accurate at discrete time or sample data points. A basis set of signal component functions is formed using the instantaneous amplitude and phase of the pilot source and related signal components observed in real data, for example, a set of bases comprising harmonics of the pilot sweep. These bases are used to form complex modulation functions for each signal component or harmonic to be analyzed and synthesized by this invention. The seismic signal being investigated is used to modulate the derived complex modulation function and the output is low pass filtered. This low pass filter output is used to compute the instantaneous amplitude and differential phase of the signal component being analyzed. From these computations the particular signal components being targeted are synthesized. These synthesized signal component data are then isolated from the original target data by subtraction, leaving the resulting time series containing the remaining signal components. Each signal component is subsequently likewise synthesized, extracted and thereby isolated in turn from the remaining data. The result is an accurate decomposition and synthesis of the original seismic signal into its various signal components.